Glossary
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earthquake
A vibration caused by the sudden breaking or frictional sliding of rock in the Earth.
earthquake belt
A relatively narrow, distinct belt of earthquakes that defines the position of a plate boundary.
earthquake engineering
The design of buildings that can withstand shaking.
earthquake zoning
The determination of where land is relatively stable and where it might collapse because of seismicity.
Earth system
The global interconnecting web of physical and biological phenomena involving the solid Earth, the hydrosphere, and the atmosphere.
ebb tide
The falling tide.
eccentricity cycle
The cycle of the gradual change of the Earth’s orbit from a more circular to a more elliptical shape; the cycle takes around 100,000 years.
ecliptic
The plane defined by a planet’s orbit.
ecosystem
An environment and its inhabitants.
eddy
An isolated, ring-shaped current of water.
effusive eruption
An eruption that yields mostly lava, not ash.
Ekman spiral
The change in flow direction of water with depth, caused by the Coriolis effect.
Ekman transport
The overall movement of a mass of water, resulting from the Eckman spiral, in a direction 90° to the wind direction.
elastic strain
A change in shape of a material that disappears instantly when stress is removed.
electromagnet
An electrical device that produces a magnetic field.
electron microprobe
A laboratory instrument that can focus a beam of electrons on a small part of a mineral grain in order to create a signal that defines its chemical composition.
El Niño
The flow of warm water east from the Pacific Ocean that reverses the upwelling of cold water along the west coast of South America and causes significant global changes in weather patterns.
elongate
cigar-shaped grains.
embayment
A low area of coastal land.
emergent coast
A coast where the land is rising relative to sea level or sea level is falling relative to the land.
end moraine (terminal moraine)
A low, sinuous ridge of till that develops when the terminus (toe) of a glacier stalls in one position for a while.
energy
The capacity to do work.
energy resource
Something that can be used to produce work; in a geologic context, a material (such as oil, coal, wind, flowing water) that can be used to produce energy.
eon
The largest subdivision of geologic time.
epeirogenic movement
The gradual uplift or subsidence of a broad region of the Earth’s surface.
epeirogeny
An event of epeirogenic movement; the term is usually used in reference to the formation of broad mid-continent domes and basins.
ephemeral (intermittent) stream
A stream whose bed lies above the water table, so that the stream flows only when the rate at which water enters the stream from rainfall or meltwater exceeds the rate at which water infiltrates the ground below.
epicenter
The point on the surface of the Earth directly above the focus of an earthquake.
epicontinental sea
A shallow sea overlying a continent.
epoch
An interval of geologic time representing the largest subdivision of a period.
equant
A term for a grain that has the same dimensions in all directions.
equatorial low
The area of low pressure that develops over the equator because of the intertropical convergence zone.
equinox
One of two days out of the year (September 22 and March 21) in which the Sun is directly overhead at noon at the equator.
era
An interval of geologic time representing the largest subdivision of the Phanerozoic eon.
erg
Sand seas formed by the accumulation of dunes in a desert.
erosion
The grinding away and removal of Earth’s surface materials by moving water, air, or ice.
erosional coast
A coastline where sediment is not accumulating and wave action grinds away at the shore.
erosional landform
A landform that results from the breakdown and removal of rock or sediment.
erratic
A boulder that was picked up by a glacier and deposited hundreds of kilometers away from the outcrop from which it detached.
esker
A ridge of sorted sand and gravel that snakes across a ground moraine; the sediment of an esker was deposited in subglacial meltwater tunnels.
estuary
An inlet in which seawater and river water mix, created when a coastal valley is flooded because of either rising sea level or land subsidence.
Eubacteria
The kingdom of “true bacteria.”
euhedral crystal
A crystal whose faces are well formed and whose shape reflects crystal form.
eukaryotic cell
A cell with a complex internal structure, capable of building multicellular organisms.
eustatic sea-level change
A global rising or falling of the ocean surface.
evaporate
To change from liquid to vapor.
evapotranspiration
The sum of evaporation from bodies of water and the ground surface and transpiration from plants and animals.
exfoliation
The process by which an outcrop of rock splits apart into onion-like sheets along joints that lie parallel to the face of the outcrop.
exhumation
The process (involving uplift and erosion) that returns deeply buried rocks to the surface.
exotic terrane
A block of land that collided with a continent along a convergent margin and attached to the continent; the term “exotic” implies that the land was not originally part of the continent to which it is now attached.
expanding Universe theory
The theory that says that the whole Universe must be expanding because galaxies in every direction seem to be moving away from us.
explosive eruptions
Violent volcanic eruptions that produce clouds and avalanches of pyroclastic debris.
external process
A geomorphologic process—such as downslope movement, erosion, or deposition—that is the consequence of gravity or the interaction between the solid Earth and its fluid envelope (air and water). Energy for these processes come from gravity and sunlight.
extinction
The death of the last members of a species so that there are no parents to pass on their genetic traits to offspring.
extinct volcano
A volcano that was active in the past but has now shut off entirely and will not erupt in the future.
extraordinary fossil
A rare fossilized relict, or trace, of the soft part of an organism.
extrusive igneous rock
Rock that forms by the freezing of lava above ground, after it flows or explodes out (extrudes) onto the surface and comes into contact with the atmosphere or ocean.
eye
The relative calm in the center of a hurricane.
eye wall
A rotating vertical cylinder of clouds surrounding the eye of a hurricane.